This invention relates generally to methods and apparatus for operating a wireless mobile station and more particularly to methods and apparatus for communicating with wireless mobile stations in which a paging channel is used to communicate to a plurality of such stations via a Code Division Multiple Access (CDMA) channel.
In a typical code division multiple access (CDMA) wireless system, a central base station transmits control information to many mobile stations by creating a multiplexed frame structure in which transmission time is divided into individual frames. Any given time slot may be devoted to information intended for an individual mobile station. This control channel structuring is often referred to as a xe2x80x9cpaging channel.xe2x80x9d
Those mobile stations which are not engaged in a conversation at any given time are in a standby mode. During the standby mode, a mobile station monitors a pre-assigned corresponding time slot of the paging channel transmitted by the base station for a paging channel message. Among other information that may be contained in the paging channel message, the mobile station may be informed whether an incoming call has arrived, to which channel the incoming call is allotted, and other information needed by the mobile station to prepare to receive the incoming call. In addition, during an assigned time slot, messages requiring action by the mobile station may be delivered.
A simplified timing diagram for a wireless mobile station operating in a CDMA system is shown in FIG. 1. To ensure receipt of a paging message, the mobile station enters an active state just prior to an assigned time slot, such as time tc During the time slot, the paging channel message may be received. However, time slots assigned to a mobile station are usually widely spaced in time. To conserve battery power, a mobile station in standby mode (i.e., not engaged in a call) enters a xe2x80x9csleep statexe2x80x9d during the time between assigned time slots. During a sleep state, most functions of the mobile station are suspended until the mobile station must xe2x80x9cwake upxe2x80x9d (e.g., become active) to receive information (for example, a paging message) contained in the next assigned time slot. In this way, the mobile station is maintained in a standby mode, alternating between a sleep state and an active state until a paging channel message is received that informs the mobile station that an incoming call has arrived or until the user of the mobile station wishes to make a call.
To maintain synchronization with the base station, the mobile station maintains timing for a Linear Feedback Shift Register (LFSR), which runs continuously at the base station. The LFSR operates as a long code (LC) generator for CDMA operation. In known systems, the LFSR is maintained by a clock derived from a main temperature-controlled crystal oscillator (TCXO) or by a high accuracy crystal. Thus, power consumption of the mobile station remains relatively high, resulting in a low battery life. It would therefore be desirable to provide if the mobile station could be maintained in a sleep state with the LFSR and crystal oscillator shut off.
There is therefore provided in one embodiment of the present invention a method for conserving power supplied to a linear feedback shift register (LFSR) long code (LC) generator, the method including steps of operating the LC generator during pre-assigned time slots; shutting off the LC generator during standby periods between the pre-assigned time slots; and priming starting states of the LC generator for synchronization with pre-assigned time slots following a standby period.
It will be appreciated that the above-described embodiment of the present invention enables power to be removed from the LFSR LC generator as well as the LFSR clock. Because the accuracy of the LFSR clock is usually derived from a system master TCXO, and because the LFSR clock operates at a relatively high rate, the TCXO can also be powered down to conserve power. A low frequency, low power clock source can then be used instead of the higher powered clock source and TCXO to maintain operation of the mobile during the mobile sleep state.